1. Field of the Invention
The invention relates to a fuel cell system provided with a fuel cell and a control method of the same.
2. Description of the Related Art
One known fuel cell has a structure in which an electrolyte membrane is sandwiched between a catalyst layer of one electrode (i.e., a cathode) and a catalyst layer of another electrode (i.e., an anode), and this structure is then sandwiched between a gas diffusion layer of the one electrode and a gas diffusion layer of the other electrode. If this fuel cell is stopped under a temperature condition of below freezing, water remaining in the fuel cell (i.e., in the gas diffusion layer, and in between the catalyst layer and the gas diffusion layer) may freeze. If the fuel cell is then activated while residual water is frozen, the supply of reaction gas to the electrolyte membrane may be inhibited. Therefore, Japanese Patent Application Publication No. 2008-140734 (JP-A-2008-140734) proposes a fuel cell system that scavenges residual water according to the temperature of the fuel cell. Also, with such a structure that scavenges residual water, there is a possibility that the performance of the fuel cell may decline due to too much moisture being removed. Therefore, Japanese Patent Application Publication No. 2007-35516 (JP-A-2007-35516) proposes a fuel cell system that estimates the dry state of the electrolyte membrane based on a voltage value of the fuel cell, and inhibits the voltage of the fuel cell from falling to 0 V or below by suppressing output current when the electrolyte membrane is dry.
If the fuel cell is activated while residual water on the cathode side is frozen, the supply of reaction gas (such as air) on the cathode side to the electrolyte membrane may be inhibited. Therefore, on the cathode side, a reduction reaction of hydrogen ions (hereinafter also referred to as “protons”) takes place instead of a water-forming reaction that takes place when generating power normally. When the consumption of protons progresses due to this reduction reaction, protons travel from the anode side to the cathode side via the electrolyte membrane. At this time, a large amount of water travels to the cathode side with the movement of protons.
Here, because water in the gas diffusion layer and in between the catalyst layer and the gas diffusion layer is frozen, water that has traveled to the cathode side with the movement of protons (i.e., electro-osmotic water) is not discharged outside through the gas diffusion layer, but instead accumulates in the catalyst layer and freezes. Typically, the catalyst layer has many pores and the electro-osmotic water accumulates in these pores. The amount of electro-osmotic water that flows gradually increases, and when it reaches an amount that exceeds the total volume of the pores in the cathode side catalyst layer, the electro-osmotic water accumulates between the electrolyte membrane and the cathode side catalyst layer and freezes. As a result, the catalyst layer on the cathode side separates from the electrolyte membrane, damaging the fuel cell.
However, in the past there has simply not been sufficient innovation with respect to this kind of problem. For example, in a structure that scavenges residual water, such as the structure described above, the problem described above may occur if scavenging is insufficient and residual water remains. Also, with a structure that estimates the dry state based on the voltage value of the fuel cell and suppresses output current when the electrolyte membrane is dry, the dry state of the electrolyte membrane is only estimated, so the existence of residual water cannot be accurately detected. As a result, not only may the problem described above occur, but output may be unnecessarily restricted.